A thyristor starter is provided with a converter configured to convert three-phase AC power of a commercial frequency to DC power, a DC reactor configured to smooth the DC power, and an inverter configured to convert the DC power supplied from the converter through the intermediary of the DC reactor to three-phase AC power of a desired frequency and supply the converted three-phase AC power to a synchronous motor through first to third AC lines. On the basis of the control on the three-phase AC power applied to the synchronous motor, it is possible to start the synchronous motor in a stopped state and drive it to rotate at a predetermined revolution speed (for example, see Japanese Patent Laying-Open No. 2003-61380 (PTD 1)).
In such thyristor starter, a ground fault detecting circuit is provided for detecting a ground fault. If a ground fault is detected by the ground fault detecting circuit, the operation of the thyristor starter will be stopped.
As a conventional ground fault detecting circuit, there is one achieved by connecting a three-phase transformer to the first to third AC lines between the thyristor starter and the synchronous motor, and configured to detect the occurrence of a ground fault on the basis of an output voltage from the three-phase transformer (for example, see Japanese Patent Laying-Open No. 2009-131048 (PTD 2), Japanese Patent Laying-Open No. 2010-130704 (PTD 3), and Japanese Patent Laying-Open No. 2011-130634 (PTD 4)).
As a conventional ground fault detecting circuit, there is another one in which one terminal of a first resistance element and one terminal of a second resistance element are connected to two input terminals of an inverter, respectively, a third resistance element is connected between the other terminals of the first and second resistance elements and a line of a ground voltage, and configured to detect the occurrence of a ground fault on the basis of a voltage across the terminals of the third resistance element (for example, see “Thyristor Starter Used in Thermal Power Station”, Mitsubishi Electric Technical Report, Vol. 67, No. 5, 1993 (NPD 1)).